Septic tanks have long been utilized as the primary means for residential sewage disposal in areas to which sewer lines have not been extended. A normal septic tank system employs only a single tank and the liquid effluent from such tank is drained through a leaching field. The primary function of the basic septic tank is to subject the solid materials in the sewage to biodegradation action of bacteria which normally develops in such tanks or which can be applied thereto. The bacteriological action normally converts the great majority of solid materials into relatively harmless soluble materials, gases and insoluble material. The insolubles remaining after the completion of the bacterial action collect in the bottom of the tank as a sludge which must be periodically removed.
The effectiveness of most septic tanks, however, is not limited by the accumulation of sludge but rather by the accumulation upon the top of the effluent of a scum or skin which comprises oily or fatty liquids and particles of untreated or partially treated solids surrounded by gases resulting in a flotation of such particles to the surface of the effluent. The existence of such scum or skin is highly deleterious to the effective action of the bacteriological degradation process in the septic tank because release of gas from the contents of the tank and the bacteriological action therein is greatly diminished.
The existence of a heavy scum or skin layer on the top of the effluent effectively prevents or at least hinders the generation of gases, which are primarily methane and carbon dioxide, by the anaerobic bacteria mainly in the sludge. The generated gases cannot escape and the high gas concentration adversely affects the bacteriological action. The result is that the tank scours, not because of the accumulation of sludge, but because the concentration of scum or skin on the surface of the tank fluid prevents effective bacteriological degradation. In severe situations, the discharge of the tank may have such a high concentration of untreated sewage as to cause spoilage of the leaching bed.
The adverse effects of scum on the surface of a sewage effluent has been well recognized in commercial and municipal sewage treatment systems to the extent that elaborate mechanical arrangements have been provided to move scrapers or buckets across the top of the effluent to remove the scum therefrom and discharge it exteriorly of the treatment tank or basin and separate from the normal effluent. Unfortunately, any type of mechanical system for removal of scum is far too expensive to be practical for typical home sewage systems; more importantly, the requirement for moving components necessarily means that constant maintenance would be required to keep the system in operable condition.
Typical mechanical systems of the type heretofore discussed may be found in U.S. Pat. No. 2,237,172 to Briggs, U.S. Pat. No. 2,337,859 to Stuller, U.S. Pat. No. 2,455,338 to Lind and U.S. Pat. No. 2,780,361 to Evans et al.
In Briggs U.S. Pat. No. 2,237,172, a sewage settling tank is provided with motor driven flights which skim scum from the sewage to the entry to an ascending chute across which a second set of motor driven flights extend to lift the scum over the lip of an open trough above the liquid level within the tank. The liquid is withdrawn through an effluent port spaced laterally from the trough, and at a level below the trough.
Stuller U.S. Pat. No. 2,337,859 discloses a tank with flights that extend through the liquid level surface defined by a weir in an outlet channel and are motor driven toward an auxiliary scum remover pipe spaced from the outlet channel. The scum remover has elongated wide-mouthed apertures in the pipewall which extend longitudinally of the pipe above the liquid surface level in one condition and can be lowered to and below that surface by rotation of the pipe around its longitudinal axis.
Lind U.S. Pat. No. 2,455,338 is similar to Stuller in general arrangement but has a float actuated scum remover pipe for driving the pipe in rotation by means of a limit switch, electric motor and gearing rather than the manual rotating means of Stuller.
Evans et al, U.S. Pat. No. 2,780,361 has a scum remover separate from the outlet port for effluent from a waste liquid treatment tank. A motor driven skimmer board is arranged to rotate about the central axis of a circular tank to carry the scum on the liquid surface to a scum-receiving pipe having a ported area parallel to its longitudinal axis. The port may be in the form of a continuous wide-mouthed slot and the pipe is arranged with adjusting means to raise or lower the port with respect to the liquid surface in the tank.
In addition to the elongate scum removers extending laterally over a substantial portion of treatment tanks, there have been removers of limited extent as shown in U.S. Pat. No. 1,188,159 to Dittrick, U.S. Pat. No. 2,439,633 to Reinhard, U.S. Pat. No. 2,717,873 to Montgomery et al, and U.S. Pat. No. 3,433,258 to Steele.
Dittrick U.S. Pat. No. 1,188,159 and Reinhart U.S. Pat. No. 2,717,873 each show a self-cleaning sanitary cistern including an exit at a lower portion and a funnel at the liquid surface to drain material from the upper surface as a skimmer. The funnel is of limited lateral extent and would not collect scum across a broad area of the liquid surface.
Montgomery et al. U.S. Pat. No. 2,717,873 discloses a clarifier and digester having an open ended, and longitudinally slotted, short pipe extending into a tank at a region remote from the primary exit of liquid.
Steele U.S. Pat. No. 3,433,258 discloses a tank which may be used as a septic tank and is provided with a stub pipe outlet having an array of circumferentially spaced longitudinal slots. The pipe has a closed end containing a central aperture and the slot array extends around the periphery to form an outlet strainer rather than a scum remover over a broad expanse of the liquid surface in the tank.
While multi-stage sewage treatment arrangements are known, particularly for community and industrial systems, these systems generally include mechanisms having the mechanical pumps, scrapers, and scum removal systems of the type discussed above. Domestic septic systems are known with multi-compartmented septic tanks and even multiple tanks connected in series. However, the inhibiting of anaerobic digestion by the development of scum which inhibits the escape of gas from the liquid has been found to progress in such serially related tanks such that each tank develops a scum at a progressively lower rate if direct flow from its preceding tank is permitted. Ultimately, the scum is passed to all tanks with the anaerobic action deteriorating in the tank first to receive the waste fluids to a point that it essentially becomes a trap for solids and the intended initial breakdown of the sewage is then transferred to the next tank of the series. When permitted to continue, ultimately the anaerobic activity in all the tanks becomes blocked. German Pat. No. 1,036,771 of Aug. 14, 1958 shows a multi-compartmented septic tank of this general construction wherein a thick layer of scum has developed on the surfaces of the first compartment and, although the sewage passing to the second compartment is drawn from the lower portion of the first compartment, it, too, develops a scum layer. The German disclosure appears to contemplate a high density scum since it would appear that the gravity overflow exit to the following compartment of the series is located at the scum surface.
My previously-issued U.S. Pat. No. 4,154,685, which is incorporated by reference herein, addresses a number of the problems heretofore set forth, and the present invention disclosed and claimed herein consists of an improvement over the system set forth in said patent.
In accordance with the above, an object of the present invention is to improve scum removal over the upper surface of the liquid in a septic tank system.
A second object of this invention is to minimize the development of scum in the initial compartment or tank of a septic system to maintain anaerobic action in that tank by passing metered quantities of scum to a next compartment for breakdown. This removal of scum from the preceding section mitigates against the inhibiting build-up in the first section while maintaining levels in the succeeding section which can be broken down by anaerobic action.
Another object is to eliminate mechanical movement of the scum removal equipment and the attendant complexity and maintenance burdens of such equipment.
A further object is to enhance the anaerobic digestion of sewage by removal of scum and the avoidance of the build-up of scum layers in septic tanks.
It is another object of the present invention to provide a new and improved scum removal system of the above-described character which comprises an improvement over the system shown in my earlier U.S. Pat. No. 4,154,685.
It is still another object of the present invention to provide a new and improved system, as above described, which is intended to provide for installations wherein severe clay, high water and/or limestone conditions exist.
It is accordingly a feature of this invention to provide an improved sewage treatment system primarily for residential use, and employing only gravity flow of the effluent through the system.
Specifically, it is a feature of the invention to provide a novel sewage treatment system which is an improvement over that shown in my U.S. Pat. No. 4,154,685.
A further feature of the preferred embodiment of the invention is to provide a residential sewage system having a plurality of serially-connected septic tanks, with each successive tank having its outlet disposed at a lower level than the preceding tank so that transfer of effluent through the tanks occurs by gravitational action, and to provide novel transfer means between each of the successive tanks which concurrently affect the removal of substantially all of the scum or skin collecting on the surface of the respective tanks solely through the suction and/or entraining action of the effluent passing through the transfer means. Additional features of the invention described and claimed herein include means for installing a sewage system wherein severe high water, clay and/or limestone conditions exist.
Another feature of this invention is to provide an improved scum-removing transfer system for effecting the transfer of effluent from one serially-connected tank to the next, characterized by the utilization of at least one transfer pipe, having at least a portion of which is horizontally disposed, as the primary transfer conduit disposed at the desired fluid level in the respective tank and provided with a closed upstream end to avoid improper liquid flow paths to the exit of the tanks. The horizontal portion of each transfer pipe includes a wide, generally horizontal opening extending laterally from one side of the pipe to the other across a substantial upper portion thereof. Preferably, the opening is located at an elevation above a horizontal plane passing through the centerline of the pipe. The accumulated scum on the liquid surface of each tank is drawn through the opening and broken up, and then discharged from the tank through the action of the liquid passing through said opening. Such action mitigates against the build-up of scum which hinders the release of gas from the contents of the tank.
Optionally, the scum-removing transfer pipes may be disposed over a broad lateral extent of the tanks with the scum collection area more extensive in the tanks earlier in the treatment sequence to overcome the greater tendency to develop scum in the initial treatment tanks.
Another optional feature of this invention is a primarily domestic septic system for all soils which includes a series of four or more sections, all of which have the above-described scum removal means such that the scum build-up in each tank is held in a range such that anaerobic activity is maintained in each tank. It has been found that for normal home installations, four sections of a thousand gallon capacity each when interconnected by the scum-removing transfer pipes reduce the material exiting the fourth section to an essentially clear liquid, the clear liquid can safely be issued to a drain field or to a trench or other fluid-conveying means for disposal.
Organic material from disposals and lavatory and toilet waste can thus be disposed of in accordance with the present invention with no need for periodic cleaning of the tanks.
Further objects, advantages and features of the invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings.